By 1911, Annie Jump Cannon was already one of the most important astronomers of her day -- even though she wasn't actually called an astronomer. Instead, she was a computer -- someone who analyzed and calculated the results of astronomical observations. Yet her analyses had led to a new way to classify stars -- a system that's still in use today.

Beginning 100 years ago, though, Cannon took her work to a new level: She began compiling the most important star catalog to date.

Cannon had been hired by Harvard College Observatory in 1896. Women couldn't be astronomers there, but they could be computers.

Cannon reworked the most popular way of classifying the stars into a simpler and more effective scheme. She based the system on a star's spectrum -- the breakdown of its light into individual wavelengths. In effect, this produced a system in which stars are classified by their surface temperature.

In 1911, Cannon was appointed as curator of the observatory's astronomical photographs -- thousands of glass plates. Over the next four years, she pored over the plates, classifying each star. Cannon herself classified about 225,000 stars in all. She then compiled the work to create the Henry Draper Catalog -- a work that's still a standard reference.

Astronomers don't use glass plates any more. But the Harvard plates remain a valuable resource -- one the observatory is working to preserve. More about that tomorrow.